In recent years, as a clean energy source, fuel cells have attracted attention. As the fuel cells, for example, there is a polymer electrolyte fuel cell. The polymer electrolyte fuel cell (hereinafter referred to as PEFC) includes a membrane-electrode assembly, and an anode separator and a cathode separator which sandwich the membrane-electrode assembly and are disposed in contact with an anode and a cathode, respectively. The membrane-electrode assembly includes the anode and the cathode (these are referred to as electrodes) each composed of a gas diffusion layer and a catalyst layer. Each gas diffusion layer has pores forming paths through which a reaction gas flows. The anode separator is provided with a fuel gas channel on its one main surface. The cathode separator is provided with an oxidizing gas channel on its one main surface. The fuel gas (hydrogen) supplied to the anode through the fuel gas channel is ionized (H+), passes through the gas diffusion layer and the catalyst layer of the anode, passes through the polymer electrolyte membrane through water, and migrates to the cathode side. Hydrogen ions reaches the cathode side and generates water in the catalyst layer of the cathode in a power generation reaction as follows:Anode side: H2→2H++2e−Cathode side: (½)O2+2H++2e−→H2OOverall Reaction: H2+(½)O2→H2O
The generated water flows into the oxidizing gas channel formed on the cathode separator in the form of steam or liquid. A part of the water generated in the cathode side migrates to the anode side (so-called flow back) and flows into the fuel gas channel. The generated water which has flowed into the oxidizing gas channel or the fuel gas channel migrates to downstream side along a flow of the oxidizing gas or the fuel gas. For this reason, there is a great variation in a water amount in localized region inside the electrode. This sometimes results in a great variation in the localized region.
As a solution to this problem, there is known a fuel cell including first channels into which gases flow and second channels from which the gases flow out, and having a configuration in which the first channel at the anode side and the second channel at the cathode side are disposed to face each other and sandwich an electrolyte layer, and the second channel at the anode side and the first channel at the cathode side are disposed to face each other and sandwich the electrolyte layer (see e.g., patent literature 1). In addition, there is known a polymer electrolyte fuel cell in which an anode gas channel and a cathode gas channel face each other so as to sandwich a membrane electrode assembly and an anode gas and a cathode gas run along each other within the channels (see e.g., patent literature 2).
In the fuel cell disclosed in patent literature 1, since the fuel gas and the oxidizing gas form a counter flow, and the channels face each other so as to sandwich the electrolyte layer, it is possible to lessen regions with a large water amount in the gas diffusion layers facing each other or regions with a small water amount in the gas diffusion layers facing each other, with the electrolyte layer sandwiched between them. As a result, it is possible to suppress a variation in a power generation amount from increasing in localized region in the electrode.
In the polymer electrolyte fuel cell disclosed in patent literature 2, since the anode gas is more humidified than the cathode gas, water diffuses from the anode gas flowing in the vicinity of an inlet of the anode gas channel, in the vicinity of an inlet of the cathode gas channel, and migrates from the anode electrode side toward the cathode electrode side, while water migrates from the cathode electrode side toward the anode electrode side, in the vicinity of an outlet of the anode gas channel. Thus, it is possible to properly control supply and discharge water in the overall fuel cell and maintain a good power generation performance of the fuel cell.
There is known a polymer electrolyte fuel cell in which an area where a wall surface of a groove forming a reaction gas channel and a reaction gas contact each other in an upstream region of a reaction gas channel is larger than that in other region, thereby suppressing a polymer electrolyte membrane from getting dried (see e.g., patent literature 3). In the polymer electrolyte fuel cell disclosed in patent literature 3, evaporation of water present on a wall inner surface or on a wall surface is promoted and thereby an amount of water evaporating from a groove wall surface side into the reaction gas increases. This makes it possible to suppress water from evaporating from the polymer electrolyte membrane side and the polymer electrolyte membrane from getting dried.
Furthermore, there is known a fuel cell in which an area of an electrolyte layer facing at least one of a fuel gas channel and an oxygen-containing gas channel is reduced, and the fuel gas channel and the oxygen-containing gas channel are arranged alternately in a direction of an electrolyte membrane surface (see e.g., patent literature 4). In the fuel cell disclosed in patent literature 4, it is possible to increase a power generation voltage in the fuel cell by suppressing the gas from permeating into an opposite electrode.